Electrochromic or photoelectrochromic device

ABSTRACT

An electrochromic or photoelectrochromic device possessing the property of changing color under the effect of an electric voltage and/or of a variation in the intensity of a light radiation, this device comprising at least one cathode and one anode, at least one of these electrodes being constituted at least in part of a transparent or translucent substrate bearing an electrically conductive coating, and an electrolyte arranged between these electrodes, and an electric circuit connecting said cathode and anode, characterized in that at least one of these electrodes carries a coating constituted of at least one nanocrystalline layer of at least one semiconductive material, having a roughness factor equal to at least 20, and a monolayer of electrically active molecules or of an electrically active polymer, said monolayer being absorbed on the surface of this coating, and in that the device contains at least one auxiliary electrically active compound, possibly dissolved in the electrolyte, having the property of being capable of being oxidized or reduced in a reversible manner.

This application is a divisional of prior application No. 09/549,443,now U.S. Pat. No. 6,426,827, filed Apr. 14, 2000 which is a continuationof prior pending application No. 09/142,733, now U.S. Pat. No. 6,067,184filed Oct. 13, 1998, which is a 371 of PCT/CH97/00109 filed Mar. 13,1997.

The present invention relates to an electrochromic orphotoelectrochromic device, particularly suitable for reversible storageand display of data and for the control of light transmission, makinguse of one or two electrodes made of semiconductor having high specificsurface area.

PREAMBLE

In order to prepare an electrochromic device in which the changing ofvisible light absorption is greater than 90% (for example from 5 to95%), and which uses molecules as electrochromic units, it is necessaryfor the surfaces to have densities reaching the value of Γ>5×10⁻⁸mol/cm² if the extinction coefficient of these molecules varies fromΔε=20'000 when changing oxidation state. Up to now, this requirement hasbeen met in the following manner when preparing electrochromic devices:

the electrochromic compound is present in solution contacting theelectrode. The requirement is fulfilled with a 0.25 M concentration anda layer thickness of the solution of 2 μm;

the electrochromic compound is electrochemically precipitated in a thinlayer on an electrode;

the electrochromic compound is polymerised or incorporated into a filmof polymer or composite material on the surface.

A description is provided hereunter of a new type of electrochromic andphotoelectrochromic devices, which, in order to attain the performancecharacteristics indicated earlier, make use of electrodes made ofnanocrystalline semiconductor having very high specific surface area onthe surface from which electrodes the electrochromic molecules areadsorbed. These devices are rapid, with switching times of less than 3 sfor an absorption change of at least 90%, and allowing for brilliantcolours to be obtained.

DESCRIPTION OF THE INVENTION

The invention relates to electrochemical systems comprising at least twoelectrodes, each of which may be transparent or opaque, and at least oneof which changes colour depending on:

I. the voltage applied between the two electrodes by a current-voltagesource;

II. the intensity of the light to which the system is exposed;

III. the combined influence of I and II.

Furthermore, the colour change engendered by the light may be

A. local: only the lit location changes colour;

B. global: the entire system changes colour, irrespective of thelocation of the lighting.

Furthermore, the colour change engendered by the external voltage maybe:

a. global: the entire system changes colour;

b. local: only the electrically addressed location changes colour(structured surface of the electrode).

This results in the following applications:

I-a: reversible electrochromic systems for the control of lighttransmission, governed by an external voltage-current source;

I-b: electrochromic systems for the reversible data display, governed byan external voltage-current source;

III-A: reversible photoelectrochromic systems for the optical writingand reversible data storage, controlled by a light beam (writing) and byan external voltage-current source (storage and deletion);

II-B: systems (filters, glazing), of which the transmission adaptsautomatically to the intensity of the light received.

All these colour changes, easy to observe with the naked eye except inthe case of III-A when data storage is on micrometric andsub-micrometric scale, correspond to chemical reactions which are welldefined on the molecular level, namely oxidations or reductions of anelectrochromic compound, usually grafted onto the whole of the surfaceof an electrode made of nanocrystalline semiconductor accessible to suchmolecules. Such an electrode is prepared by sol-gel process such as theone described in detail by Stalder and Augustynski in J. Electrochem.Soc. 1979, 126, 2007, while maintaining the relative humidity of theambient air at a value of between 50 and 80%, without a variation ofmore than 5%, during the hydrolysis of the metal alcoholate of whichmetal the oxide is being prepared. The thickness of the nanocrystallinelayer is between 0.1 and 10 μm or more, leading to a roughness factor ofbetween about 10 and 1000, for example 700, meaning that the electrodesurface area which is accessible to molecules having a typical diameterof 1 nm is 10 to 1000 times the value of the projected layer surface;e.g. 700 times. The result of this is that any change in the opticalproperties of a layer of molecules adsorbed on the surface of thesemiconductor will engender macroscopic effects amplified by theroughness factor. Accordingly, the light absorption by a monolayer ofcoloured molecules will be stronger by a factor equal to the roughnessfactor on a nanocrystalline electrode than on a flat surface.

Semiconductors which are particularly suitable for the preparation ofthe nanocrystalline electrodes must possess a large band gap. They maybe chosen from among the oxides of the elements from Group IV of theperiodic system, e.g. titanium, zirconium, or hafnium, from Group V,e.g. vanadium, niobium, or tantalum, from Group VI, e.g. chromium,molybdenum, or tungsten, or from other groups, e.g. silver, zinc,strontium, iron, or nickel. They may equally be of the perovskite type,such as SrTiO₃ or CaTiO₃.

In particular, the invention relates to an electrochromic orphotoelectrochromic device possessing the property of changing colourunder the effect of an electric voltage and/or of a variation in theintensity of a light radiation, this device comprising at least onecathode and one anode, at least one of these electrodes beingconstituted at least in part of a transparent or translucent substratebearing an electrically conductive coating, and an electrolyte arrangedbetween these electrodes, and an electric circuit connecting saidcathode and anode, this device being characterised in that at least oneof these electrodes carries a coating constituted of at least onenanocrystalline layer of at least one semiconductive material, having aroughness factor equal to at least 20, and a monolayer of electricallyactive molecules or of an electrically active polymer, said monolayerbeing adsorbed on the surface of this coating, and in that the devicecontains at least one auxiliary electrically active compound, possiblydissolved in the electrolyte, having the property of being capable ofbeing oxidised or reduced in a reversible manner.

It is in particular possible to envisage embodiments which present oneor more of the following specific features:

a. the semiconductor is a titanium-, zirconium-, hafnium-, vanadium-,niobium-, tantalum-, molybdenum-, tungsten-, zinc-, strontium-, iron-,nickel-, silver-oxide or a perovskite of the said metals;

b. the electrical circuit comprises a current-voltage source;

c. the device comprises a small auxiliary electrode in addition to theanode and the cathode;

It is in particular also possible to achieve a number of variants of theelectrochromic device according to the invention, each of these variantspresenting special features according to one of the following points:

1. variant in which the cathode carries an adsorbed monolayer of atleast one type of electrochromophoric molecules, which moleculescomprise at least one adsorbable attachment group, possibly apolymerisable or condensible group, and at least one type nelectrochromophoric group of which the property is to be colourless inthe oxidised state and coloured in the reduced state, the auxiliaryelectroactive compound being fixed at the anode in the form of anelectroactive coating, the electrolytic solution between the electrodescontaining at least one electrochemically inert salt in solution in asolvent;

2. variant according to point 1, in which the electrochromophoricmolecules comprise, as the electrochromophoric group,N,N′-dialkylbipyridinium or the diimide derivative ofnaphthalene-1,4,5,8-tetracarboxylic acid;

3. variant according to point 1, in which the electrochromophoricmolecules comprise, as the attachment group, the carboxylate,salicylate, catecholate or phosphonate group, and, if applicable, as thepolymerisable group, the vinyl or pyrrole group, or, as the condensiblegroup, the alcohol or amine group;

4. variant according to point 1, in which the cathode and anode aretransparent;

5. variant according to point 1, in which a reflective screen is placedbehind the cathode and the system is therefore opaque;

6. variant according to point 5, in which the reflective screen isconstituted of a microcrystalline semiconductor layer, saidsemiconductor layer being as indicated in point a above, deposited onthe cathode face located inside the system;

7. variant according to point 6, in which the anode is a metal plate;

8. variant according to point 7, in which the said metal is zinc;

9. variant according to point 1, in which the said electroactive coatingis constituted of a dense electrochemically deposited layer;

10. variant according to point 9, in which this dense layer is reducedPrussian blue (“Prussian white”, poly-ferrocyanide iron(II));

11. variant according to point 9, in which this dense layer is anelectroactive organic polymer;

12. variant according to point 9, in which this dense layer is acomposite material comprising an electroactive material;

13. variant according to point 1, in which the electroactive coating isconstituted of a nanocrystalline semiconductor layer, of which theroughness factor is greater than 20, on the surface of which is adsorbeda monolayer of electroactive molecules or an electroactive polymer;

14. variant according to point 13, in which the electroactive moleculesare electrochromophoric molecule comprising an adsorbable attachmentgroup, possibly a polymerisable or condensible group, and a type pelectrochromophoric group of which the property is to be colourless inthe reduced state and coloured in the oxidised state;

15. variant according to point 14, in which the said electrochromophoricmolecules comprise attachment groups and polymerisable group accordingto point 3;

16. variant according to point 7, in which the electrolytic solutionlikewise contains a metal salt of which metal the anode is constituted;

17. variant according to point 1, in which the said solvent is anelectrochemically inert liquid salt;

18. variant according to point 17, in which the said liquid salt is of1-ethyl-3 methylimidazolium- or 1-propyl-2,3-dimethylimidazoliumtrifluoromethane-sulfonate or -bis(trifluoromethylsulfonyl)amide;

19. variant according to point 1, in which the said solvent isacetonitrile, butyronitrile, glutaronitrile, methoxypropionitrile,dimethlysulfoxide, sulfolane, dimethylformamide, dimethylacetamide,N-methyl oxazolidinone, dimethyl-tetrahydro-pyrimidinone (DMPU);

20. variant according to point 1, in which the said electrochemicallyinert salt or salts are selected from among tetraalkylammonium-,1,3-dialkylimidazolium- or lithium hexafluorophosphate,-trifluoromethanesulfonate, -bis(trifluoro-methylsulfonyl)amide, or-perchlorate.

Likewise, it is possible to achieve in particular a number of variantsof the photoelectrochromic device according to the invention, each ofthese variants presenting special features according to one of thefollowing points:

21. variant in which the coloration of the device adapts automaticallyto the intensity of the light;

22. variant according to point 21, in which the anode carries ananocrystalline semiconductor layer, said semiconductor being inaccordance with point a above, of which the roughness factor is greaterthan 20, on the surface of which semiconductor a monolayer of asensitising agent is adsorbed, said sensitising agent comprising achromophoric group, an adsorbable attachment group, and possibly apolymerisable or condensible group;

23. variant according to point 21, in which the anode carries ananocrystalline semiconductor layer, said semiconductor being inaccordance with point a above, of which the roughness factor is greaterthan 20, on the surface of which semiconductor a monolayer ofelectrochromophoric molecules is adsorbed, said molecules comprising anadsorbable attachment group, an type p electrochromophoric group ofwhich the property is to be colourless in the reduced state and colouredin the oxidised state, and possibly a polymerisable or condensiblegroup;

24. variant according to point 21, in which the anode bears ananocrystalline semiconductor layer, said semiconductor being inaccordance with point a above, of which the roughness factor is greaterthan 20, lacking in adsorbed molecules;

25. variant according to point 22, in which the said sensitising agentcomprises a type p electrochromophoric group linked to the chromophoreof which the property is to be colourless in the reduced state andcoloured in the oxidised state;

26. variant according to point 22, in which the said sensitising agentand the said electrochromophoric molecules are co-adsorbed on the anode,in the proportions of 1 to 1, of 1 to 2, or of 1 to 5 or more;

27. variant according to point 21, in which the cathode carries ananocrystalline semiconductor layer, said semiconductor being inaccordance with point a above, of which the roughness factor is greaterthan 20, on the surface of which semiconductor is adsorbed a monolayerof at least one type of electrochromophoric molecules, which comprise atleast one adsorbable attachment group, at least one type nelectrochromophoric group of which the property is to be colourless inthe oxidised state and coloured in the reduced state, and possibly apolymerisable or condensible group;

28. variant according to point 21, in which the cathode does not carryany nanocrystalline semiconductor layer;

29. variant according to point 21, in which the said electroactiveauxiliary compound is an electrochemically active salt, capable oftransporting electrons between cathode and anode, dissolved in the saidsolution;

30. variant according to point 21, in which the said electroactiveauxiliary compound is a type p electrochromophoric group linked to thechromophore according to point 25, and in which the said solution onlycontains electrochemically inactive salts;

31. variant according to point 29, in which the said electrochemicallyactive salt is an type p or type n electrochromophore in solution;

32. variant according to points 22, 23, and 27, in which the saidelectrochromophoric molecules comprise attachment groups andpolymerisable groups according to point 3;

33. variant according to point 21, adapting favourably to visible light,in which the cathode is made in accordance with point 27 and in whichthe anode is made in accordance with point 22;

34. variant according to point 33, adapting favourably to visible light,made in accordance with point 29;

35. variant according to point 33, adapting favourably to visible light,made in accordance with point 31, in which the said electrochromophorein solution is of type p;

36. variant according to point 21, adapting favourably to visible light,in which the cathode is made in accordance with point 27, and in whichthe said solution is made in accordance with point 29;

37. variant according to point 36, adapting favourably to visible light,in which the anode is made in accordance with point 25;

38. variant according to point 36, adapting favourably to visible light,in which the anode is made in accordance with point 26;

39. variant according to point 21, adapting favourably to ultra-violet,in which the cathode is made in accordance with point 27, in which theanode is made in accordance with point 23, and in which the saidsolution is made in accordance with point 29;

40. variant according to point 21, adapting favourably to ultra-violet,in which the cathode is made in accordance with point 27, and in whichthe said solution is made in accordance with point 29;

41. variant according to point 21, adapting favourably to ultra-violet,in which the cathode is made in accordance with point 28, in which theanode is made in accordance with point 23 above, and in which the saidelectrochromophore in solution is of type p;

42. variant according to point 21, adapting favourably to ultra-violet,in which the cathode is made in accordance with point 28 and in whichthe solution is made in accordance with point 31 in which the saidelectrochromophore in solution is of type p;

43. variant according to point 21, which can be used favourably forreversible data storage, in which the cathode is made in accordance withpoint 27, and in which the said solution is made in accordance withpoint 30;

44. variant according to point 21, which can be used favourably forreversible data storage, in which the said solution is made inaccordance with point 30 and in which a reflective screen is placedbehind the anode.

45. variant according to point 44, in which the reflective screen ismade of a microcrystalline semiconductor layer, said semiconductor beingin accordance with point a above, deposited in the anode face locatedinside the system;

46. variant according to point 21, which can be used favourably forreversible data storage, in which the said solution is made inaccordance with point 30 and in which the cathode is made of a denselayer of electroactive material, capable to be reversibly reduced, saiddense layer being deposited on a layer of conductive plastic or glass;

47. variant according to point 46, which can be used favourably forreversible data storage, in which this dense layer is an electroactiveorganic polymer or a composite material comprising an electroactivematerial;

48. variant according to points 43 to 47, which can be used favourablyfor reversible data storage reacting to visible light, in which theanode is made in accordance with point 22;

49. variant according to points 43 to 47, which can be used favourablyfor data storage reacting to visible light, in which the anode is madein accordance with point 25;

50. variant according to points 43 to 47, which can be used favourablyfor data storage reacting to visible light, in which the anode is madein accordance with point 26;

51. variant according to points 43 to 47, which can be used favourablyfor data storage reacting to ultraviolet, in which the anode is made inaccordance with point 23;

52. variant according to point 21, in which the said solvent is anelectrochemically inert liquid salt, in accordance with point 18, oranother liquid, in accordance with point 19;

53. variant according to point 21, in which one of the electrolytes,electrochemically inactive, is selected in accordance with point 20.

The invention likewise relates to electrochromophoric compounds andsensitising agents, in particular in accordance with the followingpoints:

54. type n electrochromophoric compound according to point 1 formed ofone or more viologen groups (4,4′-dialkyl-bipyridinium) linked by one ormore alkyl chains which may include one or more phenylene groups andterminated by a phosphonate, salicylate, or catecholate group;

55. type n electrochromophoric compound according to point 54 furthercomprising a pyrrole, thiophene, vinyl, alcohol, or amine group;

56. type n electrochromophoric compound according to point 1 formed ofone or more diimide of naphthalene-1,4,5,8-tetracarboxylic acid groupslinked by one or more alkyl chains which may include one or morephenylene groups and terminated by a phosphonate, salicylate, orcatecholate group;

57. type n electrochromophoric compound according to point 56 furthercomprising a pyrrole, thiophene, vinyl, alcohol, or amine group;

58. type n electrochromophoric compound according to points 54 to 57such as those represented in the structures below;

59. type p electrochromophoric compound according to point 14, formed ofone or more triarylamine groups linked by one or more alkyl chains whichmay include one or more phenylene groups and terminated by aphosphonate, salicylate, or catecholate group;

60. type p electrochromophoric compound according to point 59 furthercomprising a pyrrole, thiophene, vinyl, alcohol, or amine group;

61. type p electrochromophoric compound according to points 59 and 60such as those represented by the molecule (8) of FIG. 11;

62. sensitising agent compound to which are linked one or more type pelectrochromophoric groups according to point 25 and of which thesensitising agent is a ruthenium complex comprising polypyridine ligandsof which at least one possesses one or more phosphonate, carboxylate,salicylate, or catecholate groups and at least one or more triarylaminegroups;

63. sensitising agent compound to which are linked one or more type pelectrochromophoric groups according to point 62 further comprising apyrrole, thiophene, vinyl, alcohol, or amine group;

64. sensitising agent compound to which are linked one or more type pelectrochromophoric groups according to points 62 and 63 such as thoserepresented by the molecules (9) and (10) of FIG. 12.

DESCRIPTION OF EXAMPLES

The description which follows, given by way of example, relates to thedrawings, in which:

FIG. 1 shows a first variant of a transparent electrochromic deviceaccording to the invention,

FIG. 2 shows a second variant of a transparent electrochromic deviceaccording to the invention,

FIG. 3 shows a third variant of a transparent electrochromic deviceaccording to the invention,

FIG. 4 shows an example of an opaque electrochromic device,

FIG. 5 shows the possibility of preparing an opaque electrochromicdevice of which the cathode is structured, for the reversible datadisplay,

FIGS. 6a to 6 d show different embodiments of photoelectrochromicdevices of the dynamic type, reacting to visible light (D-VIS) accordingto the invention,

FIGS. 7a to 7 d show different embodiments of photoelectrochromicdevices of the dynamic type, reacting to ultraviolet (D-UV) according tothe invention,

FIGS. 8a and 8 b show different embodiments of photoelectrochromicdevices of the persistent type, reacting to visible light (P-VIS) (FIG.8a) and of the persistent type reacting to ultraviolet (P-UV) (FIG. 8b),

FIG. 9 shows a photoelectrochromic device according to the invention forreversible data storage and writing,

FIG. 10 shows examples of type n electrochromic molecules formulae,

FIG. 11 shows an example of type p electrochromic molecule formulae, and

FIG. 12 shows examples of molecules formulae of sensitising agentshaving type p electrochromophore linked.

The designations of the electrodes as anode and cathode hereinafterrefer to their function in the course of the colouring process of thesystem. In the colouring processes described, the voltages applied aresuch that the cathode is polarised negatively and the anode positively.

1. Electrochromic systems (FIGS. 1 to 5)

1.1 Transparent electrochromic systems (FIGS. 1 to 3) for the control oflight transmission with an external current-voltage source (type I-a).

Such a system comprises two parallel and transparent electrodes, ofwhich the respective supports 1 and 8 are each formed from a conductiveplastic or glass plate, e.g. a glass plate covered with tin oxide 2 and7, preferably doped, or with indium and tin oxide, connected to anexternal electric circuit by means of contacts 9 (FIGS. 1 to 3).

One of the electrodes (cathode) is constituted of a transparent layer ofnanocrystalline semiconductor, e.g. titanium dioxide, of a thickness ofbetween 0.3 and 10 μm, for example 7 μm (reference 3 in FIGS. 1 to 3).Its surface comprises a monolayer of adsorbed electrochromophoricmolecules of the type of those defined in FIG. 10 (molecules (1) to (7))and shown in the form of triangles as symbols in FIGS. 1 to 3 (details 3a). These molecules comprise firstly an attachment group, secondly anelectrochromophoric group, which does not absorb visible light in theoxidised state (white triangles), but does absorb it in the reducedstate (black triangles) (type n electrochromophore), and, thirdly, andpossibly, one or more polymerisable or condensible groups. Theoxidoreduction potential of the electrochromophore between its twooxidation states must be close to the level or more negative than thelevel of the conduction band of the semiconductor, for example between 0and −0.8 V for titanium dioxide. The attachment group must allow for theadsorption of the molecule on the semiconductor. In the case of titaniumdioxide, use can be made for the electrochromophoric group of, forexample, one or more viologen (4,4′-dialkyl-bipyridinium) or diimide ofnaphthalene-1,4,5,8-tetracarboxylic acid groups and as the attachmentgroup, for example, carboxylate, salicylate, catecholate, orphosphonate. The polymerisable group, for example vinyl or pyrrole, orthe condensible group, for example amine or alcohol, must allow, onceadsorption has been effected, for the molecules to link to one anotherin such a way as to render the layer resistant to desorption.

As shown in FIGS. 1 and 2, the other electrode (anode) is formed of adense layer 6 (and detail 6 a) of transparent material of the polymerictype, reversibly oxidizible, colourless in the reduced state and,respectively, colourless (FIG. 1) or coloured (FIG. 2) in the oxidisedstate. The quantity of this material must be such that it is possible toextract at least the number of electrons necessary to reduce all theelectrochromic groups adsorbed on the cathode. This material is, forexample, poly-ferrocyanide iron(II) (“Prussian white”) which, in theoxidised state, forms “Prussian blue”, electrodeposited in accordancewith the method described by Itaya, Ataka, and Toshima in J. Am. Chem.Soc. 1982, 104, 4767.

Alternatively (FIG. 3), the anode is formed from a transparent layer 6of nanocrystalline semiconductor, analogous to the cathode, and thesurface of which bears a monolayer of adsorbed molecules (squares indetail 6 a of FIG. 3) which comprise firstly an attachment group,secondly an electrochromophoric group which does not absorb visiblelight in the reduced state (white squares), but does absorb it in theoxidised state (black squares) (type p electrochromophoric, such as themolecule (8) in FIG. 11), and, thirdly, and possibly, one or morepolymerisable or condensible groups. The oxidoreduction potential of theelectrochromophore between these two states must be close to the levelof the conduction band of the semiconductor, for example between 0 and−0.8 V for titanium dioxide. The electrochromophoric group may also bereplaced by an electroactive group, having similar electrochemicalproperties but of which the two oxidation states are colourless. In theevent of both the electrodes being nanocrystalline and carryingelectrochromophores, anode and cathode may bear both types n and p ofelectrochromophore, co-adsorbed, balance reached in any manner bydesorption and diffusion across the system in the situation in which theanode and cathode initially bear non-polymerised p and n electrochromesrespectively. In this system, in which the two electrodes are identical,with co-adsorption of the two electrochromophores, the pelectrochromophore is inert on the cathode and the n electrochromophoreis inert on the anode. However, depending on the polarisation, each ofthe electrodes can be used indifferently as anode or as cathode.

In the general situation in which the p electrochromophoric molecule ispresent on the anode, the cathode can also be lacking of nelectrochromophoric molecules. Its electroactivity is then based on thereversible insertion of small cations into the nanocrystallinesemiconductor, for example of lithium in the case of titanium dioxide.

In principle, the number of electrons delivered by the anode afteroxidation of all the p electrochromophores or after the maximumreversible insertion of cations must be comparable to the number ofelectrons consumed by the cathode after the reduction of all the nelectrochromophores. In order to adjust the initial state of the system,which must only contain electrochromophores in their colourless state,or in order to correct subsequent deviations of the system which wouldprevent this colourless state from being restored, a small auxiliaryelectrode may be provided for in the system. By means of irreversiblereduction or oxidation of the solvent or the electrolyte, electrons canbe provided to or withdrawn from the electrochromic electrodes in such away as to correct this deviation.

The space 4 between the two electrodes (FIGS. 1 to 3), being between 10and 100 μm, for example 30 μm, is filled with a solution formed eitherfrom a electrochemically inactive liquid salt, or an electrochemicallyinactive salt in solution in a solvent, with the possible addition ofglass beads 5 in order to ensure the spacing of the electrodes. Theliquid salt may be, for example, of the type ofN,N′-dialkyl-imidazolium-, N,N′-dialkyl-triazolium-, orN-alkyl-thiazolium trifluorornethane-sulfonate (triflate) or-bis(trifluoromethyl)sulfonylamide (bis-triflylamide), whether carryingor not other alkyl groups. The electrochemically inactive salt insolution in a solvent may be one of the said liquid salts or a solidsalt, for example lithium-, tetraalkylammonium-, or1,3-dialkylimidazolium bistriflylamide, -triflate, -perchlorate, or-hexafluorophosphate. The solvent is a liquid which is stable towardsthe components of the system, for example acetonitrile, butyronitrile,methoxypropionitrile, glutaronitrile, dimethylsulfoxide, sulfolane,dimethylformamide, dimethylacetamide, N-methyloxazolidinone,dimethyl-tetrahydropyrimidinone (DMPU).

Accordingly, in one embodiment of the invention, an electrochromicsystem has been prepared in accordance with the structure described, onconductive glass of SnO₂, with a cathode made of nanocrystallinetitanium dioxide, 7 μm thick, derived by theN-methyl-N′-(3-propylphosphonate)-bipyridinium bromide (molecule (1),FIG. 10), an anode of colourless electrochemically coatedpoly-ferrocyanide iron(II) and as a solution between the electrodes,lithium bis-triflylamide in a concentration of 0.3 M in glutaronitrile.The cell is sealed by an adhesive bonding agent or a heat-fusiblepolymer. When a voltage of 1 V is applied between the electrodes, theabsorbance of the system at 600 nm passes from 0 to 1.5 in 2 seconds,the appearance changing from transparent colourless to deep blue. Theprocess is reversible in the same interval of time.

In another embodiment, the same system has been prepared with anothermolecule adsorbed on the cathode: i.e. dimeric viologen provided with apropylphosphonate group (molecule (2) FIG. 10), of which the extinctioncoefficient at the maximal absorption is double that of viologen 1. Witha system such as this, under the same conditions, absorbance at 550 nmpasses from 0 to 3 in two seconds, the appearance changing fromtransparent colourless to deep blue. The process is reversible in thesame interval of time.

In another embodiment, the same system has been prepared with anothermolecule adsorbed on the cathode: i.e.bis-N,N′-[(3-carboxy-4-hydroxyphenyl)-4,4′]-bipyridinium bromide 6(molecule (3) FIG. 10). Such a system, under the same conditions, passesin the same manner from transparent yellowish to deep green.

1.2 Opaque electrochromic systems for controlling light reflection withan external current-voltage source (type I-b, FIG. 4), allowing for datadisplay.

The structure of the system is similar to that of the transparentelectrochromic system described under 1.1 (elements 2 and 5 and detail 3a identical to those of FIGS. 1 to 3).

The cathode is formed from an opaque layer of nanocrystallinesemiconductor or from a transparent layer 3 of this material, coated onits inside face with an opaque layer 3′ of microcrystallinesemiconductor, which in both cases functions as a diffusive reflector ifthe electrochromophore is in its colourless state.

The anode 8 may be made of an electrochemically oxidisible metal plate,e.g. zinc, of which the oxidised form is soluble in the solvent used,and which can be oxidised at a potential close to that of the conductionband of the semiconductor. The solution 4 between the electrodescontains a salt of the cation of the metal constituting the anode, athigh concentration.

Alternatively, the anode can be formed from a deposit of Prussian whiteon conductive plastic or glass, as described under 1.1.

In one embodiment, a system has been prepared of which the cathode isformed of nanocrystalline titanium dioxide 7 μm thick, coated onto theface located inside the electrochromic cell, with a layer ofmicrocrystalline titanium dioxide in opaque and white rutile form. Theanode is formed from a zinc plate. The solution between the electrodescontains zinc chloride in a concentration of 0.2 M in1-ethyl-3-methyl-imidazolium bis-triflylamide. The remainder of thedevice is identical to the embodiment described under 1.1. with theadsorbed molecules (1) or (2) as defined in FIG. 10. The system passesfrom a white appearance, by reflection, to a blue appearance, byreflection, in 2 seconds when the voltage applied between the electrodesrises from 0 to 1 V. The process is reversible in the same period oftime. If the two electrodes are connected in short-circuit, the systembecomes blue, the most thermodynamically stable state, with the zincoxidoreduction potential being lower than that of the viologenelectrochromophoric group under these conditions. Whether the system isin the coloured or colourless state, this state will persist for severalhours when the circuit is open.

In one of the two embodiments 1.1 or 1.2, the electrode (conductiveglass and semiconductor layer) may be engraved in such a way as todetermine conductive segments separated by insulating strips. Lateralelectrical connections allow for each of the segments to be controlledindependently. It is therefore possible to prepare a device which willallow for the display of symbols, digits, or letters (FIG. 5).

2. Photoelectrochromic systems (FIGS. 6 to 9)

The device features the property of changing its colour under the effectof light.

The system comprise two parallel transparent electrodes, the support ofwhich is constituted of a plate of conductive plastic or glass, forexample a glass plate coated with an indium and tin oxide or doped tinoxide.

Such a system comprises, as an internal source of current-voltage, aphotovoltaic electrode (references 1 a and 1 b in FIGS. 6 to 8, andreferences 1, 2, 3 in FIG. 9) (photoanode). This electrode, whichreplaces the anode described under 1.1, is prepared as for the cathode,by coating a layer of nanocrystalline semiconductor on a layer ofconductive plastic or glass. Under lighting, the photoanode produceselectrons at the potential of the conduction band cb (FIGS. 6 to 8) ofthe semi-conductor. These electrons are conducted by the externalelectric circuit 9, with or without the provision of additional voltage,as far as the cathode (references 6 a and 6 b, FIGS. 6 to 8, andreferences 6, 7, 8 in FIG. 9), in which, via the conduction band cb ofthe semiconductor, they reduce the adsorbed type n electrochromicmolecules. The electrons are returned at the photoanode by oxidation ofa reversible oxidisible molecule (donor 15, FIGS. 6 and 7), possibly atype p electrochromophore.

On the one hand, a distinction may be made between two types of systemswhich lead to colour under light effect:

D. Dynamic (FIGS. 6 and 7): the two redox systems, i.e. the adsorbed nelectrochrome and the donor are in direct or indirect electrochemicalcontact. In the case of a direct contact, the donor, in solution, may beeither a non-electrochromophoric electrochemical mediator, or an pelectrochromophore. In the case of an indirect contact, the pelectrochromophore is fixed to the anode by an adsorbable group and theelectrochemical contact is provided either by a non-electrochromophoricmediator in solution, or by a conductive polymer. The electron transfer,direct or indirect, oxidises the n electrochromophore and reduces thepossible p electrochromophore, a process opposite to that engendered bylight. The competition between these two processes results in thestationary dynamic state of the system, characterised by a certainproportion of the electro-chromophore(s) in their coloured state. Theglobal coloration then adapts automatically and very rapidly to thelighting intensity. The prior regulating which determines the responseof the system is performed by adjusting parameters which control thecompetition between the two opposed processes: concentration of themediator or of the p electrochromophore in solution, diffusion of thesespecies (by the solvent viscosity, for example), distance between theelectrodes, photovoltaic output of the photo-anode. The user can controlthe response by means of the external electric circuit.

P. Persistent (FIGS. 8 and 9): the two redox systems, i.e. the adsorbedn electrochromophore and the donor are not in electrochemical contact.It is possible either to fix the donor to the cathode by an adsorbablegroup, or, if this donor is in solution, to prevent its diffusion by amembrane between the electrodes. This type of system does not adapt tothe instantaneous lighting intensity, but changes colour as a functionof the quantity of photons received since the start of the lighting,until a maximum (saturation) has been reached, which then persists evenif the intensity subsequently drops. The system can be returned to thecolourless state (deletion) by the action of an external current-voltagesource which reverts the electrochromophore(s) to their initial state.

On the other hand, it is possible to define two types of anodephotovoltaic activity:

UV. The semiconductor of the anode is excited directly by theultraviolet light (hν UV, FIGS. 7 and 8b) (excitation of the band gap).The donor is in this case oxidised via the holes engendered in thevalency band of the semiconductor.

VIS. The semiconductor is sensitised to visible light (hν VIS, FIGS. 6and 8a) by adsorption of a dye. When the photovoltaic anode is lit, thedye carried on the anode injects electrons into the conduction band ofthe semiconductor. The dye must be chosen such that, in the state whenexcited by visible light, it is capable of injecting an electron intothis conduction band. Use may be made, for example, of a rutheniumcomplex comprising of aromatic ligands of the pyridine or poly-pyridinetype, provided with an attachment group which allows adsorption, e.g.phosphonate, carboxylate, salicylate, or catecholate.

Moreover, in order to ensure the flow of electrons from the photoanodetowards the cathode, and to optimise the system, it may be necessary toequip the external electric circuit with a current-voltage source orwith a diode.

This invention accordingly relates to all the photoelectrochromicsystems realised, by combining in every possible manner the D and Pprinciples with the UV and VIS principles.

2.1. Dynamic photoelectrochromic systems for self-adapting filters or“intelligent” glazing (II-B)

2.1.1 Systems reacting to visible light.

2.1.1.1 Anode with dye, cathode with electrochromophore (D-VIS, donor insolution, FIGS. 6a and 6 d).

The cathode is prepared as described under 1.1. The anode likewisecomprises a layer of nanocrystalline titanium dioxide, but of aroughness factor 10 to 30 times smaller than that of the cathode, insuch a way that its colour is not easily perceptible and accordinglydoes not lead to a the permanent coloration of the system. For example,for a cathode of a roughness factor equal to 1000, an anode will beprepared of which the roughness factor will be 30 to 100. If theroughness factor varies in linear fashion with the thickness, then, witha cathode of 10 μm thickness, for example, an anode will correspond of0.3 to 1 μm in thickness.

Because the quantity of the dye S of the anode is much less than thequantity of the n electrochrome of the cathode, each molecule of the dyemust carry out several electron injection cycles in order for the wholeof the electrochrome of the cathode to be reduced. An electrochemicalmediator 15 in solution must be present, in order to be capable ofreducing the oxidised dye S⁺ after each electron injection. Itsoxidoreduction potential must therefore be less than that of the dye. Inorder for the system to revert to its initial state, colourless when thelighting ceases, the electrochemical mediator in oxidised form must becapable of oxidising the reduced electrochrome. This process must beslow enough, however, that, under lighting, the stationary state reachedby the system is characterised by a high proportion of reducedelectrochrome. One practical method of adjusting the kinetics of theprocess is to vary the viscosity of the solution 4. The liquid salts aresolvents, well-suited to do this.

In a variant embodiment, use may be made of a p electrochromophore asthe electrochemical mediator (FIG. 6d).

In an example of realisation, the photovoltaic anode, formed from alayer of nanocrystalline titanium dioxide of 0.3 μm, carries as theadsorbed dye a ruthenium complex, e.g.cis-dithicyanato-bis-(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)or 4,4′4″-trimethyl-terpyridine-phosphonatoterpyridine ruthenium(II),the cathode is formed from a layer of nanocrystalline titanium dioxideof 7 μm, and carries the adsorbed electrochromophoric molecules, such asmolecule (1) as defined in FIG. 10, and the solution between theelectrodes comprises a salt of a cobalt complex, e.g.tris-phenanthroline-cobalt(II) trifluoromethanesulfonate and a lithiumsalt, for example bis-trifylamide, in a liquid salt described under 1.1,e.g. 1-ethyl-2-methyl-imidazolium bis-triflylamide. Under simulatedsunlight lighting (AM 1.5), the system becomes blue and adsorbs morethan 90% of the light at 600 nm. When the lighting ceases, the systemreverts to being colourless.

2.1.1.2. Anode with p electrochromophoric donor linked to the dye,cathode with n electrochromophore (D-VIS, FIG. 6b).

Such a system is constituted as under 2.1.1.1, with the exception of thefact that the roughness factor of the two electrodes 1 a and 6 a is inprinciple identical, being between 100 and 1500, 700 for example, andthat the dye S adsorbed on the cathode carries an p electrochromophoricgroup linked to this dye by a covalent bond.

When the photoanode is lit by visible light (hν VIS), the dye carried onthe anode injects electrons into the conducting band of thesemiconductor, and immediately oxidises the p electrochromophoric group,which becomes coloured. The electrons are conducted by the electriccircuit as far as the cathode, where, by means of the conduction band ofthe semiconductor, they reduce the adsorbed electrochromic molecule.

In principle, the number of electrons delivered by the anode after theoxidation of all the p electrochromophores must be comparable to thenumber of electrons consumed by the cathode after reduction of all the nelectrochromophores.

Such a system is naturally coloured in both the lit and the non-litstates. In the absence of lighting, its coloration results from thelight absorption by the dye; under lighting, absorptions of the oxidisedp electrochromophores and reduced n electrochromophores are added.

The solution between the electrodes is formed by an electrochemicalmediator of which the oxidoreduction potential is midway between that ofthe p electrochromophore and that of the n electrochromophore. Itsconcentration, and the proportions of the oxidised and reduced forms, inprinciple equal, are adjusted in such a way that the system reverts tothe initial colourless state when the lighting ceases. This processmust, however, be sufficiently slow that, under lighting, the stationarystate reached by the system is characterised by a high proportion of thereduced n electrochrome and the oxidised p electrochrome. One practicalway of adjusting the kinetics of the process consists of varying theviscosity of the solution. The liquid salts are solvents, well-suited todo this.

In one embodiment, the photovoltaic anode is formed from ananocrystalline titanium dioxide layer of 7 μm, which carries, as theadsorbed molecule (molecules (9) or (10) as defined in FIG. 12) aruthenium complex, e.g. bis-terpyridine ruthenium, provided with anattachment group, e.g. phosphonate, and linked to an pelectrochromophoric group, e.g. (bis(4′4″-methoxyphenyl)amino-4-phenyl(for the molecule (9)) or (bis(4′,4″-methoxyphenyl)amino-4-phenoxymethyl(for the molecule 10)). The cathode, formed from a nanocrystallinetitanium dioxide layer of 7 μm, carries a monolayer of adsorbed nelectrochromophoric molecules, for example molecule (1) as defined inFIG. 10. The solution between the electrodes comprises of a cobaltcomplex salt in form of cobalt(II) and cobalt(III), e.g.tris-phenanthroline-cobalt trifluoromethanesulfonate, and a lithiumsalt, e.g. bis-trifylamide, in a liquid salt described under 1.1, e.g.1-ethyl-2-methyl-imidazolium bis-triflylamide. Under lighting bysimulated sunlight (AM 1.5), the system passes from orange to green, andabsorbs more than 90% of the light from 600 to 750 nm. When the lightingceases, the system reverts to orange and absorption of 600 to 750 nmreverts to less than 10%.

2.1.1.3. Anode with co-adsorbed electrochromophore (D-VIS, FIG. 6b)

Such a system is constituted as under 2.1.1.2, with the exception of thefact that the p electrochromophoric group is not linked by a covalentbond to the dye, but constitutes a distinct molecule, likewise carryingan attachment group, and co-adsorbed with the dye on the photoanode. Theprocess of oxidation of the p electrochromophore by the dye is notintramolecular, as under 2.1.1.2, but intermolecular. If the dye iscapable of oxidising several co-adsorbed p electrochromophoric moleculesin succession, it is possible to adsorb more electrochromophoricmolecules than dye, which limits the coloration of the non-lit system.

The solution between the electrodes is identical to that described under2.1.1.1.

In one embodiment, the photovoltaic anode is formed of a nanocrystallinetitanium dioxide layer of 7 μm, which carries as adsorbed moleculesprimarily a ruthenium complex, e.g. bis-terpyridine ruthenium providedwith an attachment group, e.g. phosphonate, and secondly an pelectrochromophoric molecule, e.g. molecule (8) as defined in FIG. 11.The surface area concentration of the electrochromophoric molecule istwice as great as that of the dye, a proportion obtained by sensitisingthe electrode by immersion in a solution of the two molecules in a ratioof concentrations of one dye to two electrochromophores. The cathode,formed from a nanocrystalline titanium dioxide layer of 7 μm, carries amonolayer of adsorbed n electrochromophoric molecules, e.g. molecule (1)defined in FIG. 10. The solution between the electrodes is identical tothat described under 2.1.1.2. Under simulated sunlight lighting (AM1.5), the system changes from very light orange to deep blue and absorbsmore than 90% of the light from 600 to 750 nm. When the lighting ceases,the system reverts to its initial state and the absorption of 600 to 750nm reverts to less than 10%.

2.1.1.4 Anode with p electrochromophoric donor linked to the dye,cathode without electrochromophore (D-VIS FIG. 6c).

The devices described under 2.1.1.2 and 2.1.1.3 may also be realisedwith a cathode 6 b of conductive plastic or glass (transparentembodiment) or metal (opaque embodiment) which does not carry anysemiconductor layer nor adsorbed molecules. The system can thereforecontain, as electrochemical mediator, a type p electrochromophore insolution.

2.1.2 Systems reacting to ultraviolet

2.1.2.1. p Electrochromophore adsorbed at the anode, nelectrochromophore adsorbed at the cathode (D-UV, FIG. 7a)

In order to obtain a system which reacts to ultraviolet, it isappropriate to adsorb on the photoanode 1 b a monolayer ofelectrochromophoric molecules, colourless in the reduced state andcoloured in the oxidised state (p electrochromophore), and on thecathode 6 a, a monolayer of n electrochromophoric molecules. Theroughness factor of the two electrodes is in principle identical, beingbetween 100 and 1500, for example 700. In principle, the number ofelectrons delivered by the anode after oxidation of all theelectrochromophores p should be comparable to the number of electronsconsumed by the cathode after reduction of all the nelectrochromophores.

When the photoanode is lit by a close ultraviolet ray (hν UV) (350-420nm), the excitation of the electrons of the valency band of thesemiconductor in the conduction band engenders in this valency band anumber of “holes” of a potential of about 3 V, which oxidise the pelectrochromophore, which becomes coloured. The electrons are conductedby the electric circuit 9 as far as the cathode, where, via theconduction band of the semiconductor, they reduce the adsorbedelectrochromic molecule.

The solution between the electrodes is formed of an electrochemicalmediator 15, of which the oxidoreduction potential is midway betweenthat of the p electrochromophore and that of the n electrochromophore.Its concentration and the proportions of the oxidised and reduced forms,in principal equal, are adjusted in such a way that the system revertsto the initial state, colourless, when the lighting ceases. This processmust, however, be sufficiently slow that, under lighting, the stationarystate reached by the system is characterised by a high proportion of thereduced n electrochrome and the oxidised p electrochrome. One practicalmethod of adjusting the kinetics of the process is to vary the solutionviscosity. The liquid salts are solvents well-suited to do this.

In one embodiment, the photovoltaic anode is formed from ananocrystalline titanium dioxide layer of 7 μm, which carries asadsorbed p electrochromophoric molecule a triarylamine provided with anattachment group, e.g. sodium(bis-(4′,4″-methoxyphenyl)amino-4-phenoxy)-3-propylphosphonate (molecule(8) as defined in FIG. 11). The cathode, formed from a nanocrystallinetitanium dioxide layer of 7 μm, carries the adsorbed nelectrochromophore, e.g. molecule (1) as defined in FIG. 10. Thesolution between the electrodes is identical to that described under2.1.1.2.

2.1.2.2 Anode with p electrochromophoric donor, cathode withoutelectrochromophore (D-UV, FIG. 7b)

The device described under 2.1.2.1 can also be realised with a cathode 6b of conductive plastic or glass (transparent embodiment) or of metal(opaque embodiment) which does not carry any semiconductor layer noradsorbed molecules. The system can therefore contain, as electrochemicalmediator 15, a type n electrochromophore in solution.

2.1.2.3 Anode without electrochromophoric donor, cathode with nelectrochromophore (D-UV, FIG. 7c)

The device described under 2.1.2.1 can also be realised with an anode 1b of which the semiconductor does not carry any layer of adsorbedelectrochromophoric molecules. The holes in the valency band of thesemiconductor therefore oxidise directly the electrochemical mediator 15in solution, which oxidises the reduced n electrochromic compound of thecathode 6 a.

2.1.2.4 Anode without electrochromophoric donor, cathode withoutelectrochromophore, electrochemical mediator p electrochromophore insolution (D-UV, FIG. 7d)

The device described under 2.1.2.3 can also be realised with a cathode 1b of conductive plastic or glass (transparent embodiment) or of metal(opaque embodiment), which does not carry any layer of semiconductor noradsorbed molecules. The electrochemical mediator 15 is therefore an pelectrochromophore.

2.2 Persistent photoelectrochromic systems for reversible data storage(III-A) (FIGS. 8 and 9)

2.2.1 Systems reacting to visible light (P-VIS, FIGS. 8a and 9)

The device features the property of changing its colour under the effectof light when a voltage is applied to it. Whatever the lighting may be,it reverts to its initial state when the voltage is inverted.

The system comprises two transparent or opaque, parallel electrodes, asdescribed under 2.1, connected by an electric circuit provided with acurrent-voltage source 9.

2.2.1.1 Transparent systems The anode 1 a (FIG. 8a) or 1,2,3 (FIG. 9),with adsorbed molecules, is formed in a manner analogous to thatdescribed under 2.1.1.2 (adsorbed molecules of type (9) and (10) asdefined in FIG. 12, and represented in FIG. 9, detail 3 a, by arectangle (chromophore) and a square (p electrochromophore)) or that asdescribed under 2.1.1.3 (co-adsorption of the adsorbed molecules of type(8) defined in FIG. 11 and a ruthenium complex provided with anattachment group). In addition to these molecules, it may be necessaryto co-adsorb an electrochemically inactive molecule which isolates theadsorbed p electrochromophores, from one to another, in such a way as toavoid the electron transfer between these electrochromophores, whichwould prevent the local confinement of the information.

The cathode is formed as described under 1.1, with the exception of thefact that it is not coated with a molecular monolayer. The solutionbetween the electrodes 4 contains a salt of an ion which is capable ofbeing reversibly inserted within the semiconductor, dissolved in asolvent such as described under 1.1. When a light beam 10 (FIG. 9), forexample a laser, reaches an area of the anode, the dye of the siteinjects electrons into the conduction band of the semiconductor, and theoxidised dye immediately oxidises the p electrochromophoric group whichis bonded to it. Under the effect of the electrical voltage applied, theelectron is extracted from the anode and conducted to the cathode, whereit allows for the inserting of a cation.

In one variant, the cathode is prepared as described under 1.1, with theexception of the fact that it is coated with a molecular monolayer of ann electrochromophore (FIG. 8a) or of an adsorbable electroactivemolecule, of which the two oxidation states are colourless (non nelectrochromophoric element). In this case, the electron which isextracted from the anode and conducted to the cathode allows for thereduction of the electroactive molecule, whether it iselectrochromophoric or not.

In another variant (FIG. 9), the cathode is formed from a dense layer oftransparent material 6, of the polymeric and reversibly oxidisable type,colourless in both its oxidation states. In this case, the electronextracted from the anode and conducted to the cathode allows for thereduction of the polymer.

The local coloration engendered by the oxidation of theelectrochromophoric molecule at the point of lighting may be deleted bycancelling the voltage between the electrodes, and thus allowing ashort-circuit to occur, even by applying an opposed voltage.

In one embodiment, the anode is formed from a nanocrystalline titaniumdioxide layer of 7 μm, which carries as adsorbed molecule (molecule (9)or (10), FIG. 12), a ruthenium complex, e.g. bis-terpyridine ruthenium,provided with an attachment group, e.g. phosphonate, and bonded to an pelectrochromophoric group, e.g. (bis(4′,4″-methoxyphenyl)amino-4 phenyl(for the molecule (9)) or (bis(4′,4″-methoxyphenyl)amino-4 phenoxymethyl(for the molecule (10)). The cathode is formed from a nanocrystallinetitanium dioxide layer of 7 μm. The solution between the electrodescomprises a lithium salt, e.g. bis-trifylamide in a liquid salt asdescribed under 1.1, e.g. 1-ethyl-2-methyl-imidazolium bis-triflylamide.When the system is subjected to a voltage of 0.5 V, in such a way thatthe anode is charged positively and the cathode negatively, localisedlighting for one minute by white light of the intensity of 1 sun (AM1.5) produces a change of colour from orange to green exclusively at thelit portion area. The system can be maintained in an open circuit inthis state for several hours. It can be reverted to its initial state byapplying a voltage of −0.5 V.

2.2.1.2 Opaque systems

The system is identical to the transparent photoelectrochromic system asdescribed under 2.2.1, except with regard to the anode. This is formedfrom an opaque layer of nanocrystalline semiconductor.

In one embodiment, the anode is formed from nanocrystalline titaniumdioxide of 7 μm in thickness, coated onto the face located inside of thecell with a microcrystalline titanium dioxide layer in opaque and whiterutile form, which carries as the adsorbed molecule (molecules (9) or(10) FIG. 12). The remainder of the device is identical to theembodiment described under 2.2.1. This system is better compatible withthe presence of an n electrochromophore on the cathode, due to the factthat this is masked entirely by the opaque anode and that its possiblecoloration does not interfere with a data reading and storage processtaking place on the anode.

2.2.2 Systems reacting to ultraviolet (P-UV, FIG. 8b)

In the situation in which the cathode does not carry an adsorbed dye,but solely an p electrochromorphic compound, the system reacts toultraviolet in the same manner as the system described under 2.2.1reacts to visible light.

What is claimed is:
 1. A type n electrochromophoric compound, characterised in that it is formed of one or more 4.4′dialkvl-bipyridinium viologen groups linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group.
 2. A type n electrochromophoric compound according to claim 1, characterised in that it further comprises a pyrrole, thiophene, vinyl, alcohol, or amine group.
 3. A type n electrochromophoric compound, characterised in that it is formed of one or more diimide of naphthalene-1, 4,5,8-tetracarboxylic acid groups linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group.
 4. A type n electrochromophoric compound according to claim 3, characterised in that it further comprises a pyrrole, thiophene, vinyl, alcohol, or amine group.
 5. A type p electrochromophoric compound, characterised in that it is formed of one or more triarylamine groups linked by one or more alkyl chains which may include one or more phenylene groups and terminated by a phosphonate, salicylate, or catecholate group.
 6. A type p electrochromophoric compound according to claim 5, characterised in that it further comprises a pyrrole, thiophene, vinyl, alcohol, or amine group.
 7. A sensitising agent compound to which are linked one or more type p electrochromophoric groups, characterised in that the sensitising agent is a ruthenium complex comprising polypyridine ligands of which at least one possesses one or more phosphonate, carboxylate, salicylate, or catecholate groups and at least one or more triarylamine groups.
 8. A sensitising agent compound according to claim 7, characterised in that it further comprises a pyrrole, thiophene, vinyl, alcohol, or amine group. 